Long metal products are generally produced in a plant using a succession of steps. Normally, in a first step, metallic scrap is provided as feed material to a furnace, which heats the scrap liquid status. Afterwards, continuous casting equipment is used to cool and solidify the liquid metal and to form a suitably sized metal strand. Such a strand may then be cut to produce a suitably sized intermediate long product, typically a billet or a bloom, to create feed stock for a rolling mill. Normally, such feed stock is then cooled in cooling beds. Thereafter, a rolling mill is used to transform the feed stock, otherwise called billet or bloom depending on dimensions, to a final long product, for instance rebars or rods in straight products or coils, available in different sizes, which can be used in mechanical or construction industry. To obtain this result, the feed stock is pre-heated to a temperature which is suitable for its entering the rolling mill so as to be rolled by rolling mill equipment including multiple rolling stands. During rolling through these multiple stands, the feed stock is reduced to the desired cross section and shape. The long product resulting from the former rolling process is normally cut when it is still in a hot or warm condition, typically between 500 and 980° C.; cooled in a cooling bed; and finally cut at a commercial length, typically between 12 and 24 m, and packed to be ready for delivery to the customer in bundles of 1 to 5 tons.
All long metal products obtained by continuous casting and rolling exit the rolling mill with a certain speed and length. They generally need to be cut and then decelerated when advancing along a delivery path which ends with a cooling bed where the long metal products are stored for further processing and/or packaging.
For instance, hot rolled steel ribbed bars or rebars, which are typically used for concrete reinforcement. After the last rolling pass of so called high speed rolling mills, the hot rolled steel is quenched to about 500 to 600° C. and is cut to a defined length that typically is around 90 m to 120 m. From a 12 m long billet with a weight of 2 tons, a bar with a length of more than 3000 m can be generated. The speed of the steel at the rolling mill exit is normally about 30 to 50 m/s. After cutting, the bars need to be suitably braked in order to allow their unloading onto cooling beds. The bars so produced need to reach the cooling beds preferably at a speed which is close to 0.
In view of the above facts, one major technical challenge is to brake the bars from 30 m/s and above at the exit from the rolling mills to a speed suitable for unloading on cooling beds, such as for instance to 2 m/s, in the shortest time.
Current technologies perform braking of bars or, in general, of long products by motorized rotating rolls that clamp the bar to mechanically decelerate it. Magnetic equipment for braking the bar by friction between magnets and the bar itself has also been used.
According to these existing technologies, long products such as bars are pinched between two rotating rolls that, by closing on each bar for instance via a pneumatic cylinder, brake the bar. The contact pressure on the bars' surface and the friction coefficient generate a braking force on the bars.
The rotating rolls are usually mechanically connected to electric motors for the deceleration. Typical installed power is 400 to 800 kW distributed in 2 to 4 motors which are independently driven.
Due to the deformability of long products such as the above bars at temperatures that, immediately at the exit of the last rolling stand, are still around 600° C. on average, the pressure exerted by the rotating rolls for braking can result in an unacceptable deformation of the long product to the point of altering the shape of its cross section.
In order to limit the above undesirable product damage side effects caused by braking using pinch-rolls according to the state of the art, the pinching force generated by the pneumatic cylinder may be limited.
However, compromising on pinching force decreases the friction coefficient between the bar and rolls and, consequently, the transmittable torque is reduced. Reducing the applied torque diminishes the braking force and accordingly diminishes the performance of the system which is thereby limited.
Increasing the number of braking rolls, or pinch-rolls, is not cost-effective because the overall cost for equipment increases with the number of braking rolls employed, at least because more roll driving means would also be required. Under these conditions, the typical installation space necessary to receive a braking unit according to the prior art is 5 to 10 m in length.
In addition to the disadvantageous way of regulating the braking force, the technologies currently employed for braking long products exiting rolling mills have a further drawback associated with the mechanical connections between pinch-rolls and then actuating means. In practice, the response time of a braking system based on pinch-rolls is low and the order of magnitude of the resulting braking cycle is of at least 1 second.
None of the existing plants for production of long metal products by continuous casting and rolling processes manages to decelerate the long products exiting the rolling mill and to deliver them to a cooling bed, while at the same time guaranteeing that the shape and mechanical properties of the long products remain unchanged, without compromising the effectiveness of the braking effect.
Moreover, none of the existing solutions for decelerating long metal products on leaving the last rolling mill stand is specifically designed to effectively take into account at the same time
the throughput, that is, the rate at which the long metal products are manufactured and ejected from the rolling mill;
the space constraints with which the plant layout design ideally complies;
the costs of operating a manufacturing plant for continuous casting and rolling of long products provided with a relative braking system allowing storing such long products on cooling beds;
the product quality in terms of shape and technological properties.
Thus, a need exists in the prior art for a method, and a corresponding system, for decelerating long products exiting from a rolling mill, such as bars, which preserves unaltered the shape and functional characteristics of such long products that result from the rolling process, while concurrently efficiently coping with the related throughput rates and with the speed at which the long products leave the rolling mills.
A need exists in the prior art also for a method, and a corresponding system, for decelerating long products exiting a rolling mill, such as bars, which guarantees a reduction in the spaces required for arresting and then packaging such long products, while allowing costs linked to equipment and machinery.